The present invention relates to a spark timing control apparatus for use with a spark ignition internal combustion engine, and more particularly to a spark timing control system of the type wherein a variable, corresponding to the engine speed, is used to determine the spark timing. More particularly, the present invention relates to a spark timing apparatus of the above type and having a device for preventing abnormal vibrations of the engine due to fluctuations in spark timing corresponding to fluctuations in the sensed engine speed occurring for a certain time after the engine operation is switched from deceleration to acceleration.
A spark timing control apparatus has been proposed which includes a microcomputer which calculates a basic fuel injection time duration using sensed current engine speed and sensed current flow rate of intake air into the engine. The basic fuel injection time duration is the adjusted according to other parameters, such as, for example, constants determined by the temperature of the engine cooling water, the battery voltage and the output of an oxygen meter provided in the exhaust system. The microcomputer within the control apparatus then retrieves the value of spark timing determined by engine speed from an engine speed--spark timing advance angle characteristic stored in a ROM when either (1) a throttle switch is detected to be on (i.e., the throttle valve is detected to be completely closed), or determined by engine speed and fuel injection time from an engine speed-fuel injection time-spark timing advance characteristic when the throttle switch is detected to be off (i.e., the throttle valve is detected to be not completely closed). Each time the crankshaft is rotated through 360 degrees, fuel injection is effected for the calculated injection time duration and spark ignition is effected at a calculated angular position before the top dead center position each time a certain numbers of references pulses are inputted into the computer.
When the engine operation is switched from the deceleration state where the throttle valve is closed to the acceleration state where the throttle is opened, the engine speed fluctuates for a time due to twisting vibrations of the driving system, including the engine and transmission. These vibrations are created by the fact that the driving system, which has been driven so far by the vehicle body during deceleration, begins to drive the vehicle body in an opposite sense during acceleration. Thus the value of engine speed sensed by the crank angle sensor fluctuates according to the frequency inherent in the driving system.
As described above, the optimal spark timing angular value when the throttle valve is open is retrieved according to the sensed current engine speed and the corrected fuel injection time duration, which is calculated according to engine speed. Thus when the engine speed fluctuates, the corrected fuel injection time duration also fluctuates so that the spark timing retrieved from the ROM according to the sensed actual engine speed and the fuel injection time duration greatly fluctuates. When the spark timing fluctuates, the output torque of the engine fluctuates also so that with prior control systems, for a short time after the vehicle operation changes from deceleration to acceleration, the vehicle tends to fluctuate in the fore-and-aft direction of the vehicle, resulting in an uncomfortable feeling for the passagers.
It is therefore an object of the present invention to provide a spark timing control apparatus for use with a spark ignition engine which prevents abnormal vibrations of the vehicle due to fluctuations of spark timing.